The present invention relates to a frequency synthesizer and more particularly to a frequency synthesizer utilizing a direct digital frequency synthesizer.
Direct Digital Synthesis (DDS) is a well established circuit technique used in frequency synthesizers in applications where wide frequency ranges or very fast frequency hopping, or both, are required. One such synthesizer using this technique is the SP2002 400 MHz Direct Digital Frequency Synthesizer (DDFS) manufactured by GEC Plessey Semiconductors. This synthesizer chip is able to produce square, triangular and sinewave in phase and quadrature output signals at a frequency dependent on an applied clock frequency and an applied binary word. However, the levels of spurious signals on the outputs of such frequency synthesizers can be too high for use as, for example, the local oscillator in a radio receiver to be practical.
The unwanted spurious signals, or spurs, occur at amplitude and phase related terms, which are impossible to resolve on a conventional spectrum analyzer display though they can be considered as vectors. In this way, phase related spurs are orthogonal to the carrier whilst amplitude related spurs are in-phase or anti-phase with the carrier. The magnitude, number and position of the spurs in the output spectrum is related to the ROM look up table size, the non-linearities of the DACs, the amount of phase accumulator truncation and the frequency select commands of a DDFS. A discussion of these effects, along with a proposed modification to partially overcome them, is found in a paper by H. T. Nicholas and H. Samueli entitled "An Analysis of the Output Spectrum of Direct Digital Frequency Synthesisers in the Presence of Phase-Accumulator Truncation" at pp 495-502 of the published Proceedings of the 41st IEEE Annual Frequency Control Symposium.
Several other techniques exist for removing these spurious signals though each compromises the properties of the DDFS. The main applications of these DDFSs have therefore been limited to those where the spurious signals are acceptable, or where filtering by, for example, phase-locked loop techniques is possible.